As Li-ion battery technology has improved, the use of Li-ion cells for consumer electronics applications has quadrupled over the past decade. The current annual market is approaching ˜$10B. With the expected introduction of much larger Li-ion systems in applications such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and power grid energy storage systems, the total market has the potential to see an order of magnitude increase over the next two decades. Currently there is a wide range of form factors for Li-ion cells. They can generally be categorized as cylindrical cells (cells that have a hard stainless steel case with a crimped or laser welded header), prismatic cells (flat cells with a hard case and laser welded header) and pouch cells (flat cells encased in thin, aluminum laminate packaging). Cylindrical cells are commonly found in computer and other portable electronics packs. Currently flat cells, including pouch cells, are more common in smaller consumer electronics applications such as cell phones. However, larger pouch cells are increasingly being adopted for use in high-end laptop computers and in tablets, in which the lightweight laminate packaged cells are closely integrated with the device to maximize device energy density and runtime. Even larger pouch cells are being used in the most popular EV applications to take advantage of the same higher energy density and good thermal characteristics the form factor offers. With demonstrated long life the Li-ion prismatic pouch cell is positioned to dominate these emerging markets.
While pouch cells have these advantages over hard case cells, they also pose unique problems that have proven very difficult to overcome. In particular, any gas generation in the cell causes it to expand like a balloon because these cells are contained in a soft laminate, heat sealed package. Li-ion chemistries are highly reactive and gas generation in a cell can occur under many situations. Initially, this expansion can damage the pack or device in which it resides or affect the thermal management of the system. And, because Li-ion chemistries are highly sensitive to exposure to air, and the seal quality and strength is necessarily exceptionally strong, the cell may fail explosively when the pouch finally bursts. To limit this problem, many of the advances in technology that have made pouch cells more widely accepted have involved preventing or mitigating gas generation under normal operating conditions. However, excessive gas generation due to cell packaging defects or under common abuse conditions that are inherent to Li-ion chemistries, has not been overcome. Thus, the risk of a cell ballooning and explosively venting flammable electrolyte in certain situations is a major concern for battery manufacturers, OEMs and their customers. Hard case cells address the issue of venting excess gas by incorporating rupture disks in the header, such that the cell vents once a certain pressure is exceeded. A similar mechanism for the controlled release of gas built up in pouch cells to prevent ballooning or an explosion and the risk of fire has yet to be developed. As a result, pouch cells have the potential to pose a safety risk particularly under abuse conditions. This has become an even greater concern as the pouch cells being developed have become so large and energy dense.